Polycrystalline diamond and manufacturing method thereof, scribe tool, scribing wheel, dresser, rotating tool, orifice for water jet, wiredrawing die, cutting tool, and electron emission source

ABSTRACT

Nano polycrystalline diamond is composed of carbon, an element of different type which is an element other than carbon and is added to be dispersed in carbon at an atomic level, and an inevitable impurity. The polycrystalline diamond has a crystal grain size not greater than 500 nm. The polycrystalline diamond can be fabricated by subjecting graphite in which the element of different type which is an element other than carbon has been added to be dispersed in carbon at an atomic level to heat treatment within high-pressure press equipment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.15/163,298, filed May 24, 2016, which is a Divisional of U.S. patentapplication Ser. No. 14/235,763, filed Jan. 28, 2014, now issued as U.S.Pat. No. 9,714,197, which is a 371 application of InternationalApplication No. PCT/JP2012/068932, filed Jul. 26, 2012, which claims thebenefit of Japanese Patent Application Nos. 2011-165747, 2011-165748 and2011-165749, filed Jul. 28, 2011.

TECHNICAL FIELD

The present invention relates to polycrystalline diamond and amanufacturing method thereof, a scribe tool, a scribing wheel, adresser, a rotating tool, an orifice for water jet, a wiredrawing die, acutting tool, and an electron emission source, and particularly todiamond having crystal grains of a nano size, to which an element otherthan carbon has uniformly been added (hereinafter referred to as“different-type-element-added nano polycrystalline diamond”),group-III-element-added nano polycrystalline diamond,group-V-element-added nano polycrystalline diamond, and a method ofmanufacturing the former, as well as various tools and an electronemission source containing the polycrystalline diamond.

BACKGROUND ART

It has recently been clarified that a nano polycrystalline diamondsintered object has hardness exceeding natural single-crystal diamondand has a property excellent as a tool. Though the nano polycrystallinediamond is essentially an insulator, a further function such asconductivity can be provided to diamond by adding other elements such asan appropriate dopant. In addition, by appropriately selecting anelement to be added to diamond, various characteristics of diamond suchas optical characteristics, electrical characteristics, and mechanicalcharacteristics can be varied.

For example, a method of adding a dopant to graphite by forming a solidsolution thereof is available as a method of adding a dopant capable ofproviding conductivity to diamond, as shown in E. A. Ekimov et al.,Nature, Vol. 428 (2004), 542 to 545.

Though the nano polycrystalline diamond is essentially an insulator asdescribed above, conductivity can be provided to diamond by adding anelement serving as an acceptor to diamond. The document above describesa method of synthesizing diamond to which boron has been added.

In addition, conductivity can be provided to diamond by adding anelement serving as a donor to diamond. For example, Japanese PatentLaying-Open No. 2010-222165 describes a diamond layer containing anelement capable of providing conductivity. Though electrons can beemitted from n-type diamond in particular, it has been impossible toobtain n-type diamond doped with a donor at a high concentration throughhigh-temperature and high-pressure synthesis. In order to solve thisproblem, an example where doping with phosphorus is carried out withvapor phase synthesis (CVD) has been reported. With this method,however, it is extremely difficult to achieve doping at a highconcentration or to introduce a dopant other than phosphorus.

CITATION LIST Patent Document

-   PTD 1: Japanese Patent Laying-Open No. 2010-222165

Non Patent Document

-   NPD 1: E. A. Ekimov et al., Nature, Vol. 428 (2004), 542 to 545

SUMMARY OF INVENTION Technical Problem

As described above, however, with the method of adding a dopant tographite by forming a solid solution thereof, it is difficult todisperse a dopant in graphite at the atomic level. Therefore, a dopantwill unevenly distribute in graphite. When graphite in which a dopantthus unevenly distributes is directly converted to diamond, crystalgrains of diamond will become large locally in a portion where a dopantconcentration is high. Consequently, a crystal grain size of diamondwill vary approximately from several ten nm to several hundred μm.Therefore, it becomes difficult to obtain doped nano polycrystallinediamond having crystal grains all in the same nano size. In addition,when graphite in which a solid solution with a dopant is formed isdirectly converted to diamond, a dopant cluster is also produced.

In forming a solid solution of graphite with a dopant, generally, anindividual element serving as a dopant or such a compound as oxide,hydride, or halide of an element serving as a dopant is employed. Whensuch a substance is employed, however, hydride, oxide, or the like of adopant will also remain or produce a compound. Owing to a catalyticaction thereof, crystal grains of resultant diamond may locally becomeabnormally large.

A next possible method is a method of pulverizing graphite powders anddopant powders as finely as possible, carrying out strict screening,then mixing these powders, subjecting the powders further to heatingreaction treatment, and employing the resultant powders as a sourcematerial.

With this method, however, it is difficult to mix graphite and a dopantat the atomic level, and most of dopant atoms form clusters of at leasttwo or more atoms aggregated. Therefore, concentration distribution of adopant in graphite is likely, and crystal grains of diamond made ofgraphite also tend to partially grow fast. Therefore, in a case of thistechnique as well, it is difficult to obtain a doped nanopolycrystalline diamond sintered object having crystal grains having thesame nano size.

The problem described above also similarly arises in the case that anelement other than a dopant is added to diamond.

With the method described in NPD 1 above, graphite and B₄C are caused toreact with each other so as to form a solid solution of graphite withboron. In the case of this method as well, however, it is difficult todisperse boron in graphite at the atomic level. Therefore, when graphitein which a solid solution with boron is formed is directly converted todiamond, dopant clusters or the like will again be produced. Inaddition, in forming a solid solution of graphite with boron, hydride,oxide, or the like of boron is produced. Owing to a catalytic actionthereof, a crystal grain size of resultant diamond may locally becomeabnormally large. Consequently, it becomes difficult to fabricateboron-added nano polycrystalline diamond having the same crystal grainsize.

Then, a next possible method is exemplified by a method of pulverizinggraphite powders and powders of such an acceptor element as boron asfinely as possible, carrying out strict screening, then mixing thesepowders or subjecting the powders further to heating reaction treatment,and employing the resultant powders as a source material, as in the casedescribed above.

With this method again, however, it is difficult for acceptor atomsalone to mix with graphite powders, and most of the acceptor atoms formclusters in which at least two or more atoms are adjacent to each other.Therefore, concentration distribution of the acceptor element in diamondis likely. Consequently, crystal grains of diamond tend to partiallygrow fast, and it has been difficult to obtain nano polycrystallinediamond having crystal grains of a uniform nano size.

On the other hand, n-type conductivity can be provided to diamond byadding an element serving as a donor to diamond. Diamond having n-typeconductivity has electron emission characteristics as described above.By making use of these characteristics, diamond having n-typeconductivity can be used, for example, for an electron gun.

Single crystal synthesis or vapor phase synthesis has been known as atechnique for fabricating diamond having conductivity. Whichevertechnique of single crystal synthesis and vapor phase synthesis may beemployed, however, it is very difficult to fabricate nanopolycrystalline diamond having a donor element uniformly added theretoand having n-type conductivity.

For example, with a technique of directly converting graphite todiamond, it has been considered that diamond can be doped with a donorbecause donor atoms can be confined in diamond. Actually, however, it isextremely difficult to form a uniform solid solution of graphite, whichis a source material, with donor atoms.

A method of adding donor atoms to diamond in a most simplified manner isexemplified by a method of pulverizing graphite powders and powders of adonor element as finely as possible, carrying out strict screening, thenmixing these powders or subjecting the powders further to heatingreaction treatment, and employing the resultant powders as a sourcematerial. With this method, however, it is difficult for donor atomsalone to mix with graphite powders, and most of the donor atoms formclusters in which at least two or more atoms are adjacent to each other.Therefore, concentration distribution of the donor element in diamond islikely. Consequently, crystal grains of diamond tend to partially growfast, and it has been difficult to obtain nano polycrystalline diamondhaving crystal grains of a uniform nano size.

The present invention was made in view of the problems as describedabove, and one object of the present invention is to provide nanopolycrystalline diamond obtained by uniformly adding an element otherthan carbon to diamond and a manufacturing method thereof.

Another object of the present invention is to provide nanopolycrystalline diamond obtained by uniformly adding an acceptor elementto diamond at an unprecedented level and a manufacturing method thereof,a scribe tool, a scribing wheel, a dresser, a rotating tool, an orificefor water jet, a wiredrawing die, and a cutting tool including thepolycrystalline diamond.

Yet another object of the present invention is to provide nanopolycrystalline diamond obtained by uniformly adding a donor element todiamond and a manufacturing method thereof and an electron emissionsource made of the polycrystalline diamond.

Solution to Problem

Polycrystalline diamond according to the present invention is composedof carbon, an element of different type which is an element other thancarbon and is added to be dispersed in carbon at the atomic level, andan inevitable impurity. The polycrystalline diamond has a crystal grainsize approximately not greater than 500 nm.

The element of different type is preferably dispersed in carbon as asubstitutional isolated atom. A concentration of the element ofdifferent type is, for example, approximately not lower than 1×10¹⁴/cm³and not higher than 1×10²²/cm³. The polycrystalline diamond can befabricated by sintering graphite obtained by thermally decomposing a gasmixture of a gas containing the element of different type and ahydrocarbon gas at a temperature not lower than 1500° C.

A method for manufacturing polycrystalline diamond according to thepresent invention includes the steps of preparing graphite that anelement of different type which is an element other than carbon is addedto be dispersed in carbon at the atomic level and directly convertingthis graphite to diamond by subjecting graphite to heat treatment withinhigh-pressure press equipment.

In the step of converting graphite to diamond, preferably, graphite isheated within the high-pressure press equipment without adding asintering aid or a catalyst. The step of preparing graphite may includethe step of forming graphite to which the element of different type hasbeen added on a base material by introducing a gas mixture of a gascontaining the element of different type and a hydrocarbon gas within avacuum chamber and thermally decomposing the gas mixture at atemperature not lower than 1500° C. In the step of converting graphiteto diamond, graphite formed on the base material may be heated withinthe high-pressure press equipment. The gas mixture is preferably fedtoward a surface of the base material. For example, a methane gas can beused as the hydrocarbon gas.

Polycrystalline diamond according to another aspect of the presentinvention is composed of carbon, a group III element added to bedispersed in carbon at the atomic level, and an inevitable impurity. Thepolycrystalline diamond has a crystal grain size (a maximum length of acrystal grain) approximately not greater than 500 nm.

The group III element is preferably dispersed in carbon as asubstitutional isolated atom. A concentration of the group III elementis, for example, approximately not lower than 1×10¹⁴/cm³ and not higherthan 1×10²²/cm³. The polycrystalline diamond can be fabricated bysintering graphite obtained by thermally decomposing a gas mixture of agas containing the group III element and a hydrocarbon gas at atemperature not lower than 1500° C.

A method for manufacturing polycrystalline diamond according to anotheraspect of the present invention includes the steps of preparing graphitethat a group III element is added to be dispersed in carbon at theatomic level and converting this graphite to diamond by subjectinggraphite to heat treatment within high-pressure press equipment.

In the step of converting graphite to diamond, preferably, graphite issubjected to heat treatment within the high-pressure press equipmentwithout adding a sintering aid or a catalyst. The step of preparinggraphite may include the step of forming graphite on a base material byintroducing a gas mixture of a gas containing the group III element anda hydrocarbon gas within a vacuum chamber and thermally decomposing thegas mixture at a temperature not lower than 1500° C. In the step ofconverting graphite to diamond, graphite formed on the base material maybe subjected to heat treatment at a high pressure not lower than 8 GPaand at 1500° C. or higher. The gas mixture is preferably fed toward asurface of the base material. For example, a methane gas can be used asthe hydrocarbon gas.

The polycrystalline diamond can be used in various tools, Specifically,the polycrystalline diamond can be used for a scribe tool, a scribingwheel, a dresser, a rotating tool, an orifice for water jet, awiredrawing die, and a cutting tool.

Polycrystalline diamond according to yet another aspect of the presentinvention is composed of carbon, a group V element added to be dispersedin carbon at the atomic level, and an inevitable impurity. Thepolycrystalline diamond has a crystal grain size approximately notgreater than 500 nm.

The group V element is preferably dispersed in carbon as asubstitutional isolated atom. A concentration of the group V element is,for example, approximately not lower than 1×10¹⁴/cm³ and not higher than1×10²²/cm³. The polycrystalline diamond can be fabricated by sinteringgraphite obtained by thermally decomposing a gas mixture of a gascontaining the group V element and a hydrocarbon gas at a temperaturenot lower than 1500° C.

A method for manufacturing polycrystalline diamond according to yetanother aspect of the present invention includes the steps of preparinggraphite that a group V element is added to be dispersed in carbon atthe atomic level, which has a crystal grain size not greater than 10 μm,and converting this graphite to diamond by subjecting graphite to heattreatment within high-temperature and high-pressure press equipment.

In the step of converting graphite to diamond, preferably, graphite issubjected to heat treatment within the high-temperature andhigh-pressure press equipment without adding a sintering aid or acatalyst. The step of preparing graphite may include the step of forminggraphite on a base material by introducing a gas mixture of a gascontaining the group V element and a hydrocarbon gas within a vacuumchamber and thermally decomposing the gas mixture at a temperature notlower than 1500° C. In the step of converting graphite to diamond,graphite formed on the base material may be subjected to heat treatmentwithin the high-temperature and high-pressure press equipment. The gasmixture is preferably fed toward a surface of the base material. Forexample, a methane gas can be used as the hydrocarbon gas.

An electron emission source according to the present invention is madeof the polycrystalline diamond described above.

Advantageous Effects of Invention

In the polycrystalline diamond according to the present invention, sincean element of different type is added to be dispersed in carbon at theatomic level, an element of different type can be added to diamond withuniformity at an unprecedented level.

In the method for manufacturing polycrystalline diamond according to thepresent invention, since conversion to polycrystalline diamond iscarried out by subjecting such graphite that an element of differenttype which is an element other than carbon is added to be dispersed incarbon at the atomic level to heat treatment within a vacuum chamber,polycrystalline diamond to which an element of different type hasuniformly been added at an unprecedented level can be fabricated.

In the polycrystalline diamond according to another aspect of thepresent invention, since a group III element is added to be dispersed incarbon at the atomic level, a group III element can be added to diamondwith uniformity at an unprecedented level and p-type conductivity can beprovided to nano polycrystalline diamond.

In the method for manufacturing polycrystalline diamond according toanother aspect of the present invention, since conversion topolycrystalline diamond is carried out by subjecting such graphite thata group III element is added to be dispersed in carbon at the atomiclevel to heat treatment within a vacuum chamber, nano polycrystallinediamond to which a group III element has uniformly been added at anunprecedented level can be fabricated and p-type conductivity can beprovided to nano polycrystalline diamond.

Since the polycrystalline diamond according to the present inventionalso has excellent resistance to oxidation, it is useful for such toolsas a scribe tool, a scribing wheel, a dresser, a rotating tool, anorifice for water jet, a wiredrawing die, and a cutting tool.

In the polycrystalline diamond according to yet another aspect of thepresent invention, since a group V element is added to be dispersed incarbon at the atomic level, a group V element can be added to diamondwith uniformity at an unprecedented level and n-type conductivity can beprovided to the polycrystalline diamond.

In the method for manufacturing polycrystalline diamond according to yetanother aspect of the present invention, since conversion topolycrystalline diamond is carried out by subjecting such graphite thata group V element is added to be dispersed in carbon at the atomiclevel, which has a crystal grain size not greater than 10 μm, to heattreatment within high-temperature and high-pressure press equipment,polycrystalline diamond to which a group V element has uniformly beenadded at an unprecedented level can be fabricated. Though particles ofat least several microns have conventionally segregated, according tothe present invention, a group V element can uniformly be added todiamond to such a level as not allowing distinction with resolution ofseveral microns and to such an extent that segregation of a compoundother than diamond cannot be ascertained even with such high-intensityX-ray facilities as Spring 8, that is, such that a group V element ispresent as a substitutional element at a carbon position substantiallyat the atomic level. In addition, n-type conductivity can be provided tothe polycrystalline diamond and the polycrystalline diamond has electronemission characteristics.

Since the electron emission source according to the present invention ismade of the polycrystalline diamond described above, it has both ofexcellent electron emission characteristics and durability.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view showing a state that polycrystallinediamond in one embodiment of the present invention is fabricated on abase material.

FIG. 2 is a perspective view showing a state that polycrystallinediamond in another embodiment of the present invention is fabricated ona base material.

FIG. 3 is a diagram showing one example of distribution of an impurityin boron-added nano polycrystalline diamond in another embodiment of thepresent invention.

FIG. 4 is a perspective view showing a state that polycrystallinediamond in yet another embodiment of the present invention is fabricatedon a base material.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be described hereinafterwith reference to FIG. 1.

Different-type-element-added nano polycrystalline diamond in the presentembodiment contains an element of different type added to be dispersedat the atomic level in carbon forming a body of the polycrystallinediamond. Here, an “element of different type” herein refers to anelement which can be added to diamond, is other than carbon formingdiamond, and is not an inevitable impurity contained in diamond. As anelement of different type, for example, nitrogen, hydrogen, a group IIIelement, a group V element, silicon, a metal such as a transition metal,rare earth, and the like can be exemplified. It is noted that a singleelement of different type alone can be added to diamond and a pluralityof elements of different type may simultaneously be added to diamond.

As shown in FIG. 1, nano polycrystalline diamond 1 in the presentembodiment is formed on a base material 2 and contains an element ofdifferent type 3 uniformly dispersed at the atomic level. It is notedthat “dispersed at the atomic level” herein refers, for example, to adispersed state at such a level that, when carbon and an element ofdifferent type are mixed in a vapor phase state and solidified tothereby fabricate solid carbon in a vacuum atmosphere, the element ofdifferent type is dispersed in solid carbon. Namely, this state is sucha state that an element precipitated as isolated or a compound otherthan diamond is not formed.

Nano polycrystalline diamond 1 can be fabricated by subjecting graphiteformed on the base material to heat treatment. Graphite is an integralsolid and contains a crystallized portion. Though polycrystallinediamond 1 has a flat-plate shape in the example in FIG. 1, it ispossible to have any shape and thickness. In the case that nanopolycrystalline diamond 1 is fabricated by subjecting graphite formed onthe base material to heat treatment, nano polycrystalline diamond 1 andgraphite basically have the same shape.

The element of different type above can be added to graphite in thestage of formation of graphite. Specifically, graphite can be formed onthe base material by thermally decomposing a gas mixture of a gascontaining the element of different type and a hydrocarbon gas at atemperature not lower than 1500° C. so that at the same time, theelement of different type can be added to graphite. Thus, by mixing theelement of different type in a source material gas for formation ofgraphite in a vapor phase state to thereby add the element of differenttype to graphite, the element of different type can uniformly be addedto graphite at the atomic level. In addition, by appropriately adjustingan amount of addition of a gas containing the element of different typeto the hydrocarbon gas, a desired amount of element of different typecan uniformly be added at the atomic level.

The gas mixture can thermally be decomposed within a vacuum chamber, andby setting a degree of vacuum within the vacuum chamber to be relativelybe high here, introduction of an impurity into graphite can besuppressed. Actually, however, an unintended inevitable impurity isintroduced in graphite. An element such as nitrogen, hydrogen, oxygen,boron, silicon, and a transition metal, other than the element ofdifferent type above, can be exemplified as this inevitable impurity.

In graphite used for fabricating different-type-element-added nanopolycrystalline diamond in the present embodiment, an amount of eachinevitable impurity is approximately 0.01 mass % or lower. Namely, aconcentration of an inevitable impurity in graphite is approximately nothigher than a detection limit in SIMS (Secondary Ion Mass Spectrometry)analysis. In addition, a concentration of a transition metal in graphiteis approximately not higher than a detection limit in ICP (InductivelyCoupled Plasma) analysis or SIMS analysis.

Thus, in the case that an amount of an impurity in graphite is lowereddown to a level of the detection limit in SIMS analysis or ICP analysisand diamond is made of this graphite, diamond extremely small in anamount of impurity other than an element of different type, of whichaddition has been intended, can be fabricated. It is noted that, evenwhen graphite containing an impurity slightly more than the detectionlimit in SIMS analysis or ICP analysis is employed, diamond havingcharacteristics significantly better than in a conventional example isobtained.

Nano polycrystalline diamond in the present embodiment uniformlycontains an element of different type at the atomic level as above andan amount of an impurity therein is also extremely small. In the nanopolycrystalline diamond, atoms of the element of different type do notaggregate as clusters in carbon but they are substantially uniformlydispersed over the entire diamond. Ideally, atoms of the element ofdifferent type are present as isolated from one another in carbon.

As described above, since nano polycrystalline diamond in the presentembodiment contains the element of different type dispersed in carbon atthe atomic level, nano polycrystalline diamond to which the element ofdifferent type has uniformly been added at an unprecedented level isobtained. In addition, since the element of different type can uniformlybe dispersed in nano polycrystalline diamond at the atomic level,desired characteristics and functions can effectively be provided todiamond. For example, by adding an appropriate element, mechanicalcharacteristics of diamond can be improved; for example, wear resistanceof diamond can effectively be enhanced. Electrical characteristics ofdiamond can also be improved; for example, conductivity can be providedto diamond. In addition, optical characteristics of diamond can also beimproved, for example, by uniformly coloring the diamond.

For example, nitrogen can be selected as an element of different type.In this case, nitrogen can be dispersed in diamond at the atomic level.Namely, nitrogen atoms can be introduced in diamond as isolated. Here,nitrogen atoms are present in carbon (the diamond body) in a statesubstituted for carbon atoms. Namely, nitrogen atoms are not simplyintroduced in carbon but in such a state that nitrogen atoms and carbonatoms are chemically bonded to each other.

Normally, in the case that graphite containing nitrogen as a gas inpores is directly converted to diamond at such a high temperature as2300° C. and such a high pressure as 20 GPa, nitrogen of approximatelyseveral hundred ppm is introduced in diamond as aggregated, and isolatednitrogen in diamond is approximately 1 ppm or lower. This isolatednitrogen is important for coloring of diamond, and diamond containingisolated nitrogen exhibits a color from yellow to orange. When thisdiamond is irradiated with electron beams and heated at a hightemperature not lower than 600° C., coloring of red or pink is observed,which means that, through the treatment as above, a defect absorbinglight around 550 nm and emitting light around 638 nm, which is called anNV (Nitrogen Vacancy) defect which is combination of nitrogen and adefect, has been generated. This NV defect is not generated withoutnitrogen present as isolated and as a substitutional atom. On the otherhand, nitrogen introduced in diamond as aggregated does notsubstantially contribute to coloring of diamond.

In nano polycrystalline diamond in the present embodiment to whichnitrogen has been added, since nitrogen is dispersed in diamond at theatomic level, there is substantially no nitrogen introduced asaggregated in diamond. Therefore, when nano polycrystalline diamond isirradiated with electron beams and heated at a high temperature notlower than 600° C., diamond can be colored to red or the like. Inaddition, added nitrogen does not aggregate at a crystal grain boundaryof diamond and there is very little impurity in diamond. Therefore,abnormal growth of a diamond crystal can also effectively be suppressed.Consequently, colored nano polycrystalline diamond is obtained, althoughit is a polycrystalline body having a crystal grain size (a maximumlength of a crystal grain) from 10 to 500 nm.

In addition, in nano polycrystalline diamond in the present embodiment,since such an element of different type as nitrogen described above isdispersed in the diamond body at the atomic level, concentrationdistribution of the element of different type in diamond is also lesslikely. From this fact as well, local abnormal growth of diamond crystalgrains can effectively be suppressed. Consequently, as compared with aconventional example, sizes of crystal grains of diamond can also be thesame.

A concentration of an element of different type in diamond canarbitrarily be set. A high or low concentration of an element ofdifferent type can be set. In any case, since an element of differenttype can be dispersed in diamond at the atomic level, generation ofconcentration distribution of the element of different type in diamondcan effectively be suppressed. It is noted that a concentration of anadded element of different type is preferably within a rangeapproximately from 10¹⁴ to 10²²/cm³ in total, in order to maintain acrystal grain size of polycrystalline diamond within a rangeapproximately from 10 to 50 nm.

A method for manufacturing different-type-element-added nanopolycrystalline diamond in the present embodiment will now be described.

Initially, in a vacuum chamber, a base material is heated to atemperature approximately not lower than 1500° C. and not higher than3000° C. A well known technique can be adopted as a heating method. Forexample, it is possible that a heater capable of directly or indirectlyheating the base material to a temperature not lower than 1500° C. isprovided in the vacuum chamber.

Any metal, inorganic ceramic material, or carbon material can be used asthe base material, so long as it is a material capable of withstanding atemperature approximately from 1500° C. to 3000° C. From a point of viewof not introducing an impurity in graphite serving as a source material,however, the base material is preferably made of carbon. Morepreferably, it is possible that the base material is composed of diamondor graphite containing very little impurity. In this case, at least asurface of the base material should only be composed of diamond orgraphite.

Then, a hydrocarbon gas and a gas containing an element of differenttype are introduced in the vacuum chamber. Here, a degree of vacuumwithin the vacuum chamber is set approximately to 20 to 100 Torr. Thus,the hydrocarbon gas and the gas containing the element of different typecan be mixed within the vacuum chamber, and graphite in which theelement of different type has been taken at the atomic level can beformed on the heated base material. In addition, in the case that acompound is introduced as a source of an element of different type aswell, an unnecessary component does not remain. It is noted that thebase material may be heated after introduction of the gas mixture andthen graphite containing the element of different type may be formed onthe base material.

For example, a methane gas can be used as the hydrocarbon gas. A gas ofa hydride or an organic compound of an element of different type ispreferably adopted as the gas containing the element of different type.By adopting a hydride as the element of different type, the hydride ofthe element of different type can readily be decomposed at a hightemperature. Alternatively, by adopting an organic compound as theelement of different type, such a state that the element of differenttype is surrounded by carbon, that is, the element of different type isisolated from each other, can be established. Thus, the element ofdifferent type as isolated is readily taken into graphite.

When nitrogen is selected as an element of different type, for example,a gas of methyl amine or an analogue thereof can be employed. In thecase that a methane gas and a methyl amine gas are bubbled in an argongas to make a gas mixture, the gas mixture can be introduced in thevacuum chamber at a ratio from 10⁻⁷% to 100%.

In forming graphite, the hydrocarbon gas and the gas containing theelement of different type are preferably fed toward the surface of thebase material. Thus, the gases can be mixed efficiently in the vicinityof the base material, so that graphite containing the element ofdifferent type can efficiently be generated on the base material. Thehydrocarbon gas and the different-type-element-containing gas may besupplied from directly above the base material toward the base material,or may be supplied toward the base material in an oblique direction orin a horizontal direction. It is also possible that a guide member forguiding the hydrocarbon gas and the different-type-element-containinggas to the base material is provided in the vacuum chamber.

Graphite that an element of different type which is an element otherthan carbon is added to be dispersed in carbon at the atomic level,which is manufactured as described above, is sintered in high-pressurepress equipment, so that different-type-element-added nanopolycrystalline diamond to which the element of different type hasuniformly been added at an unprecedented level can be fabricated.Namely, after sintering of graphite, nano polycrystalline diamond havingcrystal grains of a nano size is obtained. For example, thepolycrystalline diamond can have a crystal grain size approximately from10 to 500 nm.

It is noted that, in the step of converting graphite to diamond,graphite is preferably subjected to heat treatment at a high pressurewithout adding a sintering aid or a catalyst. In addition, in the stepof converting graphite to diamond, graphite formed on the base materialmay be subjected to heat treatment within ultra-high-pressure equipment.

With the method in the present embodiment, even an element which isnormally difficult to add to diamond can be confined as isolated withindiamond crystals, owing to abrupt generation of crystals.

Graphite which can be used for fabrication of nano polycrystallinediamond in the present embodiment is, for example, crystalline graphitepartially containing a crystallized portion or polycrystalline. Densityof graphite is preferably higher than 0.8 g/cm³. Thus, volume changeduring sintering of graphite can be made smaller. From a point of viewof making volume change during sintering of graphite smaller andimproving yield, experimentally, density of graphite is furtherpreferably approximately not lower than 1.4 g/cm³ and not higher than2.0 g/cm³.

The reason why density of graphite is within the range above is becauseit is considered that, when density of graphite is lower than 1.4 g/cm³,volume change during a high-temperature and high-pressure process is toolarge and temperature control may become impossible. In addition, it isbecause, when density of graphite is higher than 2.0 g/cm³, probabilityof occurrence of crack in diamond may be twice or higher.

Examples of the present invention will now be described.

Example 1

A methane gas and methyl amine were mixed at 1:1 in the vacuum chamber,and the gas mixture above was blown onto a diamond base material heatedto 1900° C. Here, a degree of vacuum within the vacuum chamber was setto 20 to 30 Torr. Then, graphite containing nitrogen deposited on asubstrate. Bulk density of this graphite was 2.0 g/cm³.

Graphite above was converted to diamond at a synthesis temperature of2300° C. and at 15 GPa, to thereby obtain nano polycrystalline diamondto which nitrogen was added. The polycrystalline diamond had a crystalgrain size from 10 to 200 nm. When this polycrystalline diamond wasirradiated with electron beams and annealed at a high temperature of800° C., red nano polycrystalline diamond was obtained.

Example 2

A methane gas and trimethyl amine were mixed at 1:1 in the vacuumchamber, and the gas mixture above was blown onto a diamond basematerial heated to 1900° C. Here, a degree of vacuum within the vacuumchamber was set to 20 to 30 Torr. Then, graphite containing nitrogendeposited on a substrate. Bulk density of this graphite was 2.0 g/cm³.As a result of ICP element analysis, a concentration of nitrogen ingraphite was 100 ppm.

Graphite above was converted to diamond at a synthesis temperature of2300° C. and at 15 GPa, to thereby obtain nano polycrystalline diamondto which nitrogen was added. The polycrystalline diamond had a crystalgrain size from 10 to 200 nm. When this polycrystalline diamond wasirradiated with electron beams and annealed at a high temperature of800° C., light red nano polycrystalline diamond was obtained.

Example 3

A methane gas and methyl amine were mixed at 1:1 in the vacuum chamber,and the gas mixture above was blown onto a diamond base material heatedto 1900° C. Here, a degree of vacuum within the vacuum chamber was setto 100 Torr. Then, graphite containing nitrogen deposited on asubstrate. Bulk density of this graphite was 2.0 g/cm³. As a result ofICP element analysis, a concentration of nitrogen in graphite was 100ppm.

Graphite above was converted to diamond at a synthesis temperature of2300° C. and at 15 GPa, to thereby obtain nano polycrystalline diamondto which nitrogen was added. The polycrystalline diamond had a crystalgrain size from 10 to 200 nm. When this polycrystalline diamond wasirradiated with electron beams and annealed at a high temperature of800° C., red nano polycrystalline diamond was obtained.

Example 4

A methane gas and trimethyl amine were mixed at 1:1 in the vacuumchamber, and the gas mixture above was blown onto a diamond basematerial heated to 1900° C. Here, a degree of vacuum within the vacuumchamber was set to 100 Torr. Then, graphite containing nitrogendeposited on a substrate. Bulk density of this graphite was 2.0 g/cm³.As a result of ICP element analysis, a concentration of nitrogen ingraphite was 150 ppm.

Graphite above was converted to diamond at a synthesis temperature of2300° C. and at 15 GPa, to thereby obtain nano polycrystalline diamondto which nitrogen was added. The polycrystalline diamond had a crystalgrain size from 10 to 200 nm. When this polycrystalline diamond wasirradiated with electron beams and annealed at a high temperature of800° C., light red nano polycrystalline diamond was obtained.

Comparative Example 1

Commercially available graphite was sealed in a nitrogen atmosphere andnano polycrystalline diamond was synthesized directly from graphiteunder a high-temperature and high-pressure condition of 2300° C. and 15GPa. Then, a concentration of nitrogen in the polycrystalline diamondwas 100 ppm. Even when this polycrystalline diamond was irradiated withelectron beams and annealed at 800° C., however, diamond did not havered color, which means that isolated nitrogen in diamond was verylittle, which was approximately 1 ppm or lower.

In Examples above, it could be confirmed that, by setting a degree ofvacuum in the vacuum chamber to 20 to 100 Torr, mixing the hydrocarbongas and a gas containing nitrogen within the vacuum chamber, andsupplying the gas mixture onto the base material heated to a temperaturearound 1900° C., graphite having a solid phase and bulk density around2.0 g/cm³, in which nitrogen had been dispersed at the atomic level,could be fabricated on the base material. In addition, it could also beconfirmed that, by converting graphite to diamond at a synthesistemperature of 2300° C. and at 15 GPa, nano polycrystalline diamondhaving a crystal grain size (a maximum length of a crystal grain)approximately from 10 to 200 nm, in which nitrogen was dispersed at theatomic level, could be fabricated. It is considered, however, that nanopolycrystalline diamond having excellent characteristics could befabricated within the scope described in Scope of Claims for Patent eventhough conditions are out of the range above.

An embodiment of another type of the present invention will be describedhereinafter with reference to FIGS. 2 to 3.

Group-III-element-added nano polycrystalline diamond in the presentembodiment contains a group III element which is added to be dispersedat the atomic level in carbon forming a polycrystalline diamond body. Inaddition, nano polycrystalline diamond in the present embodiment ispolycrystalline diamond not containing a binder and having a crystalgrain size of a nano size.

A group III element is an element which can have a bond smaller by 1 inthe number of electrons than carbon and it is an element serving as anacceptor in diamond. For example, boron, aluminum, gallium, indium,thallium, and the like can be exemplified as group III elements. Thoughone or more elements selected from these elements can be employed, otherelements having a similar function may be employed. Among the group IIIelements, boron is suitable, however, mixed elements which arecombination of boron and another element can also be employed.

As shown in FIG. 2, nano polycrystalline diamond 1 in the presentembodiment is formed on base material 2 and contains a group III element3 a uniformly dispersed at the atomic level. It is noted that the “groupIII element dispersed at the atomic level” herein refers, for example,to a dispersed state at such a level that, when carbon and a group IIIelement are mixed in a vapor phase state and solidified to therebyfabricate solid carbon in a vacuum atmosphere, the group III element isdispersed in the solid carbon.

Nano polycrystalline diamond 1 can be fabricated by subjecting graphiteformed on the base material to heat treatment at a high temperature anda high pressure. Graphite is an integral solid and contains acrystallized portion. Though nano polycrystalline diamond 1 has aflat-plate shape in the example in FIG. 2, it is possible to have anyshape and thickness. In the case that nano polycrystalline diamond 1 isfabricated by subjecting graphite formed on the base material to heattreatment, nano polycrystalline diamond 1 and graphite basically havethe same shape.

The group III element above can be added to graphite in the stage offormation of graphite. Specifically, graphite can be formed on the basematerial by thermally decomposing a gas mixture of a gas containing agroup III element and a hydrocarbon gas at a temperature not lower than1500° C. so that at the same time, the group III element can be added tographite.

As the gas containing the group III element above, for example, any of afirst gas composed of a hydride of a group III element, a second gas ofan organic metal base which is composed of a gas of one or more selectedfrom trimethylboron, triethylboron, and trimethyl borate, a third gas ofan organic metal base which is composed of a gas of one or more selectedfrom trimethylaluminum, triethylaluminum, dimethyl aluminum hydride, andtriisobutylaluminum, a fourth gas of an organic metal base which iscomposed of a gas of one or more selected from trimethylgallium andtriethylgallium, a fifth gas of an organic metal base which is composedof a gas of one or more selected from trimethylindium andtriethylindium, and a sixth gas of an organic metal base which iscomposed of a gas of one or more selected from trimethylthallium andtriethylthallium can be employed. It is also possible that two or moreof the gases above are mixed as appropriate.

As described above, by mixing a group III element in a source materialgas for formation of graphite in a vapor phase state to thereby add thegroup III element to graphite, the group III element can uniformly beadded to graphite at the atomic level. In addition, by appropriatelyadjusting an amount of addition of a gas containing the group IIIelement to the hydrocarbon gas, a desired amount of group III elementcan be added to graphite.

The gas mixture can thermally be decomposed in a vacuum chamber, and bysetting a degree of vacuum within the vacuum chamber to be relatively behigh here, introduction of an impurity into graphite can be suppressed.Actually, however, an unintended inevitable impurity is introduced ingraphite. An element such as nitrogen, hydrogen, oxygen, silicon, and atransition metal, other than the group III element above of whichaddition has been intended, can be exemplified as this inevitableimpurity.

In graphite used for fabricating group-III-element-added nanopolycrystalline diamond in the present embodiment, an amount of eachinevitable impurity is approximately 0.01 mass % or lower. Namely, aconcentration of an inevitable impurity in graphite is approximately nothigher than the detection limit in SIMS (Secondary Ion MassSpectrometry) analysis. In addition, a concentration of a transitionmetal in graphite is approximately not higher than the detection limitin ICP (Inductively Coupled Plasma) analysis or SIMS analysis.

Thus, in the case that an amount of an impurity in graphite is lowereddown to a level of the detection limit in SIMS analysis or ICP analysisand diamond is made of this graphite, polycrystalline diamond extremelysmall in an amount of impurity other than a group III element, of whichaddition has been intended, can be fabricated. It is noted that, evenwhen graphite containing an impurity slightly more than the detectionlimit in SIMS analysis or ICP analysis is employed, polycrystallinediamond having characteristics significantly better than in theconventional example is obtained.

FIG. 3 shows one example of distribution of boron and an impurity in thenano polycrystalline diamond in the present embodiment. It is noted thatboron-added nano polycrystalline diamond shown in FIG. 3 was obtained bysubjecting graphite containing boron representing the group III elementdescribed above to heat treatment at 2000° C. in a vacuum atmosphere of10⁻² Pa. In addition, a boron concentration or an impurity concentrationwas measured in SIMS analysis.

As shown in FIG. 3, it can be seen that variation in a direction ofdepth, of a boron concentration and a concentration of each impurity indiamond is less. In addition, it can be seen that an amount of animpurity in the nano polycrystalline diamond in the present embodimentis at an extremely low value.

Table 1 below shows results of comparison of a ratio of introduction ofB₄C, between boron-added nano polycrystalline diamond in the presentembodiment and boron-added nano polycrystalline diamond obtained bymixing B₄C serving as a boron source and graphite fabricated with theconventional technique, subjecting the mixture to heat treatment at2000° C. in a vacuum atmosphere of 10⁻² Pa, and forming a solid solutionof graphite with boron.

TABLE 1 Boron Concentration B₄C Introduction Ratio (Amount of MixedBoron Present Inventive Preparation) for Doping Example 0.05% 0.01%<0.01% 0.10% 0.02% <0.01% 0.25% 0.05% <0.01% 0.50% 0.10% <0.01% 1.00%0.20% <0.01% 5.00% 1.00% <0.01%

As shown in Table 1, it can be seen that, in boron-added nanopolycrystalline diamond obtained by forming a solid solution of graphitewith boron, as a concentration of added boron increases, a ratio ofintroduction of B₄C becomes higher, whereas in boron-added nanopolycrystalline diamond in the present embodiment, even though aconcentration of added boron increases, a ratio of introduction of B₄Cis extremely low, that is, lower than 0.01 mass %.

Nano polycrystalline diamond in the present embodiment uniformlycontains a group III element at the atomic level as above, while anamount of an impurity is extremely small. In this nano polycrystallinediamond, atoms of the group III element do not aggregate as clusters incarbon, but they are substantially uniformly dispersed over the entirediamond. Ideally, atoms of the group III element are present as isolatedfrom one another in carbon. Atoms of the group III element are presentin carbon (the diamond body) in a state substituted for carbon atoms,and they are not simply introduced in carbon but in such a state thatatoms of the group III element and carbon atoms are chemically bonded toeach other.

As described above, since nano polycrystalline diamond in the presentembodiment contains the group III element dispersed in carbon at theatomic level, nano polycrystalline diamond to which the group IIIelement has uniformly been added at an unprecedented level is obtained.In addition, since the group III element can uniformly be dispersed innano polycrystalline diamond at the atomic level, desired p-typeconductivity can be provided to the entire diamond.

In nano polycrystalline diamond in the present embodiment to which agroup III element has been added, since the group III element isdispersed in diamond at the atomic level, there is substantially nogroup III element introduced in diamond as aggregated as describedabove. In addition, the added group III element does not aggregate at acrystal grain boundary of diamond and there is very little impurity indiamond. Therefore, abnormal growth of a diamond crystal can alsoeffectively be suppressed. Consequently, nano polycrystalline diamondhaving a crystal grain size (a maximum length of a crystal grain) of anano size such as from 10 to 500 nm and having p-type conductivity isobtained.

Furthermore, in nano polycrystalline diamond in the present embodiment,concentration distribution of the group III element in diamond is alsoless likely. From this fact as well, local abnormal growth of crystalgrains of diamond can effectively be suppressed. Consequently, ascompared with a conventional example, sizes of crystal grains of diamondcan also be the same.

A concentration of a group III element in diamond can arbitrarily beset. A high or low concentration of a group III element in diamond canbe set. In any case, since a group III element can uniformly bedispersed in diamond, generation of concentration distribution of thegroup III element in diamond can effectively be suppressed. Thus,occurrence of local variation of conductivity in diamond can alsoeffectively be suppressed.

It is noted that a concentration of an added group III element ispreferably within a range approximately from 10¹⁴ to 10²²/cm³, in orderto provide p-type conductivity to diamond. In order to provide goodconductivity like a metal to diamond, a concentration of an added groupIII element is preferably not lower than approximately 10¹⁹/cm³, and inorder to provide a property as a semiconductor to diamond, aconcentration of an added group III element is approximately from 10¹⁴to less than 10¹⁹/cm³.

Though there were concerns about lowering in hardness as compared withnon-doped nano polycrystalline diamond by addition of a group IIIelement as above, hardness was comparable. Namely,group-III-element-added nano polycrystalline diamond in the presentembodiment has hardness equivalent to that of non-doped nanopolycrystalline diamond and has p-type conductivity.

When nano polycrystalline diamond in the present embodiment was used tofabricate, for example, a cutting tool, it was found that the cuttingtool had cutting performance and life (wear resistance characteristics)equal to or higher than those of non-doped nano polycrystalline diamond,while it had conductivity. In addition, the cutting tool also had suchan advantage that adhesion of swarf due to static electricity wassuppressed because nano polycrystalline diamond had conductivity, evenwhen an insulating material was cut. Applications of nanopolycrystalline diamond in the present embodiment will be described indetail later.

A method for manufacturing group-III-element-added nano polycrystallinediamond in the present embodiment will now be described.

Initially, in a vacuum chamber, a base material is heated to atemperature approximately not lower than 1500° C. and not higher than3000° C. A well known technique can be adopted as a heating method. Forexample, it is possible that a heater capable of directly or indirectlyheating the base material to a temperature not lower than 1500° C. isprovided in the vacuum chamber.

Any metal, inorganic ceramic material, or carbon material can be used asthe base material, so long as it is a material capable of withstanding atemperature approximately from 1500° C. to 3000° C. From a point of viewof not introducing an impurity in graphite serving as a source material,however, the base material is preferably made of carbon. Morepreferably, it is possible that the base material is composed of diamondor graphite containing very little impurity. In this case, at least asurface of the base material should only be composed of diamond orgraphite.

Then, a hydrocarbon gas and a gas containing a group III element areintroduced in the vacuum chamber. Here, a degree of vacuum within thevacuum chamber is set approximately to 20 to 100 Torr. Thus, thehydrocarbon gas and the gas containing the group III element can bemixed within the vacuum chamber. By thermally decomposing this gasmixture at a temperature not lower than 1500° C., graphite in which thegroup III element has been taken at the atomic level can be formed onthe base material. It is noted that the base material may be heatedafter introduction of the gas mixture and then graphite containing thegroup III element may be formed on the base material.

For example, a methane gas can be used as the hydrocarbon gas. Variousgases described above can be employed as the gas containing the groupIII element. The gas mixture of the hydrocarbon gas and the gascontaining the group III element can be introduced in the vacuum chamberat a ratio, for example, from 10⁻⁷% to 100%.

In forming graphite, the hydrocarbon gas and the gas containing thegroup III element are preferably fed toward the surface of the basematerial. Thus, the gases can be mixed efficiently in the vicinity ofthe base material, so that graphite containing the group III element canefficiently be generated on the base material. The hydrocarbon gas andthe group-III-element-containing gas may be supplied from directly abovethe base material toward the base material, or may be supplied towardthe base material in an oblique direction or in a horizontal direction.It is also possible that a guide member for guiding the hydrocarbon gasand the group-III-element-containing gas to the base material isprovided in the vacuum chamber.

Graphite that a group III element has been added to be dispersed incarbon at the atomic level, which is manufactured as described above, issintered at a high temperature and at a high pressure with the use ofhigh-pressure press equipment or the like, so thatgroup-III-element-added nano polycrystalline diamond to which the groupIII element has uniformly been added at an unprecedented level can befabricated.

It is noted that, in the step of converting graphite to diamond,graphite is preferably subjected to heat treatment at a high pressurewithout adding a sintering aid or a catalyst. In addition, in the stepof converting graphite to diamond, graphite formed on the base materialmay be subjected to heat treatment at a high temperature not lower than1500° C. and at a high pressure not lower than 8 GPa.

Graphite which can be used for fabrication of nano polycrystallinediamond in the present embodiment is, for example, crystalline graphitepartially containing a crystallized portion or polycrystalline. Densityof graphite is preferably higher than 0.8 g/cm³. Thus, volume changeduring sintering of graphite can be made smaller. From a point of viewof making volume change during sintering of graphite smaller andimproving yield, experimentally, density of graphite is furtherpreferably approximately not lower than 1.4 g/cm³ and not higher than2.0 g/cm³.

The reason why density of graphite is within the range above is becauseit is considered that, when density of graphite is lower than 1.4 g/cm³,volume change during a high-temperature and high-pressure process is toolarge and temperature control may become impossible. In addition, it isbecause, when density of graphite is higher than 2.0 g/cm³, probabilityof occurrence of crack in diamond may be twice or higher.

Applications of nano polycrystalline diamond in the present embodimentwill now be described.

In p-type diamond to which such an acceptor as boron has been added, apart of tetravalent carbon is occupied by trivalent atoms. In this case,though one covalently bonded electron is short in a system, an electronis externally obtained and thus a diamond structure can be maintained.

On the other hand, in a case of n-type diamond, when donors areactivated and electrons are emitted, those electrons occupy an energystate created by a conduction band, that is, created by the band offormation of antibonding orbital. Therefore, it is considered thatdiamond bond tends to be weaker.

In contrast, in the case of an acceptor, it is considered that bondingstrength of diamond is higher because non-bonding covalent bond iscompletely satisfied as a result of reception of one electron. Namely,by adding an acceptor to diamond, improvement in heat resistancecharacteristics or wear resistance of diamond can be expected.

For example, in boron-added nano polycrystalline diamond in the presentembodiment, since boron can be dispersed in diamond at the atomic level,even when a large amount of boron which is an element of different typeis added to diamond, mechanical characteristics of diamond are not atall impaired. Therefore, nano polycrystalline diamond in the presentembodiment has Knoop hardness as high as that of nano polycrystallinediamond which is non-doped and binderless. Such nano polycrystallinediamond is useful for tools used in various types of machining.

The inventors of the present application fabricated a cutting tool byusing boron-added nano polycrystalline diamond fabricated with thetechnique in the present embodiment and containing boron by 0.1 mass %,0.3 mass %, or 0.6 mass %, and actually cut an aluminum material. Then,even after cutting by 30 km, an amount of wear of a flank face was assmall as 3 μm or less. In contrast, a cutting tool was fabricated withnon-doped nano polycrystalline diamond and similar tests were conducted.Then, an amount of wear of a flank face was at least twice as much asthat of a case of boron-added nano polycrystalline diamond above.

In addition, boron-added nano polycrystalline diamond in the presentembodiment, polycrystalline diamond to which boron has been added withthe conventional technique, non-doped nano polycrystalline diamond, andsingle-crystal diamond were prepared, and they were polished with agrindstone of #1500 at 250 rpm for 30 minutes, with load of 2.5 kg beingapplied. Then, it could be confirmed that an amount of wear ofboron-added nano polycrystalline diamond in the present embodiment wasextremely small, that is, from 0.01 to 0.02 μm/minute. It is noted thatan amount of wear of other diamonds including non-doped nanopolycrystalline diamond was at least 5 times as much as an amount ofwear of boron-added nano polycrystalline diamond in the presentembodiment.

From the foregoing, it could be confirmed that excellent wear resistancewas obtained by fabricating a tool with boron-added nano polycrystallinediamond in the present embodiment.

Since nano polycrystalline diamond in the present embodiment hasconductivity, it can also be applied to electric discharge machining.More specifically, nano polycrystalline diamond in the presentembodiment can be applied to a tool for fabricating a complicatedthree-dimensional shape such as a concavely curved surface.

For example, when boron is added to polycrystalline diamond, in order toobtain a conductor applicable to electric discharge machining, aconcentration of boron to be added to diamond is preferably within arange approximately from 10²⁰ to 10²²/cm³. Here, if a concentration ofadded boron is higher than 10²²/cm³, there are concerns about difficultyin achieving homogenous dispersion of boron in diamond and impairment ofmechanical characteristics of polycrystalline diamond. Therefore, aconcentration of boron to be added to diamond as above is preferably nothigher than 10²²/cm³.

The inventors of the present application also found that, by addingboron to nano polycrystalline diamond with the technique in the presentembodiment, heat resistance in an oxygen-containing atmosphere(resistance to oxidation) could be improved as compared with non-dopednano polycrystalline diamond to which no element of different type wasadded.

When the present inventors of the present application fabricatedboron-added nano polycrystalline diamond containing boron by 0.1 mass %,0.3 mass %, or 0.6 mass % with the technique in the present embodiment,prepared non-doped nano polycrystalline diamond as a ComparativeExample, and measured an amount of remaining diamond after heattreatment for 1 hour at a temperature from 500° C. to 900° C., an amountof remaining boron-added nano polycrystalline diamond in the presentembodiment was at least 4 times as high as an amount of remainingnon-doped nano polycrystalline diamond. Namely, it could be confirmedthat boron-added nano polycrystalline diamond in the present embodimenthad heat resistance excellent in an oxygen-containing atmosphere(resistance to oxidation).

In the case that polycrystalline diamond is used for a cutting tool,even when a cutting edge of the tool is cooled with a cutting fluid ormist during cutting, a temperature of the cutting edge of the tool isclose to 1000° C. due to sliding with respect to a work material at acutting point. Therefore, in the case that polycrystalline diamond isused for a cutting tool, excellent resistance to oxidation is alsorequired.

In the case of boron-added nano polycrystalline diamond according to thepresent embodiment, when it is exposed to a high temperature in anoxygen-containing atmosphere, a film of an oxide of boron is formed on asurface of nano polycrystalline diamond. Therefore, boron-added nanopolycrystalline diamond according to the present embodiment hasresistance to oxidation better than normal polycrystalline diamond.Therefore, nano polycrystalline diamond in the present embodiment isuseful also for a cutting tool, and with the use of nano polycrystallinediamond, working of a complicated shape can also be achieved. Inaddition, tool life can also be extended, and a cutting tool capable ofworking with high shape accuracy being maintained can be provided.

Furthermore, in boron-added nano polycrystalline diamond above, sinceboron has been dispersed in diamond at the atomic level, a film of anoxide of boron can substantially uniformly be formed on the entiresurface of diamond exposed to a high temperature in an oxygen-containingatmosphere. Therefore, resistance to oxidation is extremely high and theeffect described above is noticeable.

In using insulating nano polycrystalline diamond for a tool, a method ofworking diamond is restricted. More specifically, a method for preciselyfinishing a cutting edge to a level allowing use as a tool substantiallyhas to rely on mechanical polishing. Therefore, insulating nanopolycrystalline diamond is applicable only to a tool having atwo-dimensional shape. Specifically, a shape of a tool is limited to arectangular cutting tool or a V-shaped cutting tool formed only of flatsurfaces manufactured also with single-crystal diamond, an R cuttingtool formed with a flat surface and one curved surface, or the like. Fora rotating tool as well, a shape is limited to a simple shape formedwith flat surfaces, such as a ball end mill having a cutting edge shapemade by cutting a part of an arc of a planar R cutting tool and a drillhaving four corners of a quadrangular pyramid as cutting edges.

In contrast, in the case of conductive nano polycrystalline diamond inthe present embodiment, since it is excellent in resistance tooxidation, various methods of polishing a cutting edge can be adopted.Therefore, it is applicable to a cutting tool or a rotating tool in ashape more complicated than in the case of insulating nanopolycrystalline diamond.

On the other hand, with increase in area of an optical element, a sizeof a mold has increased and demands for tools high in wear resistanceand capable of continuous cutting of a mold of a large area have alsoincreased. Since nano polycrystalline diamond in the present embodimentis excellent also in wear resistance, it is also applicable to such atool.

Single-crystal diamond has conventionally been employed for a materialfor an orifice for water jet. In single-crystal diamond, however, anamount of wear is different depending on its crystal orientation (unevenwear), and hence an intended cutting width cannot be obtained with lapseof time of use.

For example, an amount of wear is significantly different between a(111) plane and a (100) plane of single-crystal diamond, while anorifice made of single-crystal diamond has planes of various crystalorientations in a circumferential direction of an inner surface of anorifice hole. Therefore, even though the inner surface of the orificehole has a cylindrical shape at the time when use was started, as theorifice is used, wear of a surface which is likely to wear proceeds in ashort period of time, the cylindrical shape loses its shape, and theshape of the inner face of the orifice hole expands from the cylindricalshape to a polygonal shape such as a hexagonal shape. Consequently, sucha problem that an intended cutting width as described above cannot beobtained arises.

As measures against uneven wear above, use of sintered diamond ispossible. Sintered diamond is fabricated by sintering diamond particleswith the use of a metal binder such as cobalt, and the metal binder ispresent among diamond particles. A portion of the metal binder, however,is softer than the diamond particles. Therefore, wear proceeds again ina short period of time. When the binder decreases, diamond particlesalso fall off, the orifice hole is expanded, and there is a problem thata cutting width stable for a long period of time cannot be obtained. Inparticular, in the case of a water jet aiming to improve cuttingefficiency, a solution obtained by adding hard particles (alumina or thelike) to water is injected at a high pressure. Therefore, the portion ofthe metal binder softer than the diamond particles wears in a shortperiod of time and there is a problem that a cutting width stable for along period of time cannot be obtained.

For polycrystalline diamond not containing a metal binder, a method ofcoating an inner surface of an orifice hole made of metal with a thindiamond film with a CVD (chemical vapor deposition) method is available.For such reasons as being a thin film or low bonding strength betweendiamond particles, however, wear tends to proceed and life is short.

In contrast, since nano polycrystalline diamond in the presentembodiment is polycrystalline and excellent also in wear resistance,uneven wear as described above can effectively be suppressed. Inaddition, since nano polycrystalline diamond in the present embodimentcontains no binder, progress of wear in a binder portion can also beavoided. Therefore, nano polycrystalline diamond in the presentembodiment is useful also for an orifice for water jet.

As applications of a diamond material other than the above, an exampleof use of single-crystal diamond as a material for a wiredrawing die canbe exemplified.

In single-crystal diamond, however, the problem of uneven wear asdescribed above arises, and therefore, in a wiredrawing die includingsingle-crystal diamond, there is a problem that an intended wirediameter and circularity cannot be obtained with lapse of time of use.

In the case of the wiredrawing die made of single-crystal diamond aswell, as in the case of the orifice for water jet, an inner surface of adie hole has planes in various crystal orientations in a circumferentialdirection. Therefore, even though the inner surface of the die hole hasan annular shape at the time when use was started, wear of a surfacewhich is likely to wear proceeds in a short period of time, and theshape of the inner face of the die hole expands from the annular shapeto a polygonal shape such as a hexagonal shape. Consequently, there is aproblem that an intended wire diameter and circularity cannot beobtained.

As measures against uneven wear above, use of sintered diamond is alsopossible, however, as in the case of the orifice for water jet, a diehole expands and there is a problem that a wire diameter and circularitystable for a long period of time cannot be obtained.

Then, use of an acid for removal of a binder for use is considered.Since bonding strength among diamond particles is low, however, diamondparticles fall off, and again there is a problem that an intended wirediameter and circularity cannot be obtained.

Use of polycrystalline diamond with the CVD method which ispolycrystalline diamond containing no binder is also possible, however,there is a problem that bonding strength among diamond particles isagain low, wear tends to proceed, and life is short. Similarly, it isalso possible to use polycrystalline diamond containing no binder, whichwas obtained by direct conversion sintering with the use of indirectheating at an ultra-high pressure not lower than 8 GPa and an ultra-hightemperature not lower than 2200° C., with high-purity graphite servingas a starting material. In this case, however, resistance to oxidationis poor, and hence there is a problem that wear tends to proceed andlife is short.

In addition, in the case of fabrication of a wiredrawing die having ahole in a polygonal shape such as a quadrangular shape other than anannular shape, single-crystal diamond is worked with laser and highlyprecise working has been difficult. Though diamond containing a metalbinder is conductive and capable of electric discharge machining,diamond has a large particle size, which similarly makes highly preciseworking difficult.

In contrast, since nano polycrystalline diamond in the presentembodiment can effectively suppress uneven wear and contains no binder,even progress of wear in a binder portion can be avoided. In addition,since nano polycrystalline diamond in the present embodiment has greatbonding strength among diamond particles, also has conductivity, and isexcellent also in resistance to oxidation, it is useful also for awiredrawing die.

Similarly, nano polycrystalline diamond in the present embodiment isuseful also for such tools as a scribe tool, a scribing wheel, and adresser.

Examples of another type of the present invention will now be described.

Example 5

A methane gas and trimethylboron were mixed at 1:1 in the vacuumchamber, and the gas mixture above was blown onto a diamond basematerial heated to 1900° C. Here, a degree of vacuum within the vacuumchamber was set to 20 to 30 Torr. Then, graphite containing borondeposited on a substrate. Bulk density of this graphite was 2.0 g/cm³.As a result of ICP element analysis, a concentration of boron ingraphite was 0.06 mass %.

Graphite above was converted to diamond at a synthesis temperature of2200° C. and at 15 GPa, to thereby obtain nano polycrystalline diamondto which boron was added. The polycrystalline diamond had a crystalgrain size (a maximum length of a crystal grain) from 10 to 100 nm. Noprecipitation of B₄C was observed in X-ray patterns. This nanopolycrystalline diamond had Knoop hardness of 120 GPa. A substratehaving a size of 3 mm×1 mm was cut from the nano polycrystalline diamondand electrical resistance of the substrate was measured, which was 100Ω.

Example 6

A methane gas and trimethyl borate were mixed at 1:1 in the vacuumchamber, and the gas mixture above was blown onto a diamond basematerial heated to 1900° C. Here, a degree of vacuum within the vacuumchamber was set to 20 to 30 Torr. Then, graphite containing borondeposited on a substrate. Bulk density of this graphite was 2.0 g/cm³.As a result of ICP element analysis, a concentration of boron ingraphite was 0.5 mass %.

Graphite above was converted to diamond at a synthesis temperature of2200° C. and at 15 GPa, to thereby obtain nano polycrystalline diamondto which boron was added. The polycrystalline diamond had a crystalgrain size from 10 to 100 nm. No precipitation of B₄C was observed inX-ray patterns. This nano polycrystalline diamond had Knoop hardness of120 GPa. A substrate having a size of 3 mm×1 mm was cut from the nanopolycrystalline diamond and electrical resistance of the substrate wasmeasured, which was 10Ω.

Example 7

A methane gas and trimethyl borate were mixed at 1:1 in the vacuumchamber, and the gas mixture above was blown onto a diamond basematerial heated to 1900° C. Here, a degree of vacuum within the vacuumchamber was set to 20 to 30 Torr. Then, graphite containing borondeposited on a substrate. Bulk density of this graphite was 2.0 g/cm³.As a result of ICP element analysis, a concentration of boron ingraphite was 0.5 mass %.

Graphite above was converted to diamond at a synthesis temperature of2200° C. and at 8 GPa, to thereby obtain nano polycrystalline diamond towhich boron was added. The polycrystalline diamond had a crystal grainsize from 10 to 100 nm. No precipitation of B₄C was observed in X-raypatterns. This nano polycrystalline diamond had Knoop hardness of 120GPa. A substrate having a size of 3 mm×1 mm was cut from the nanopolycrystalline diamond and electrical resistance of the substrate wasmeasured, which was 10Ω.

Example 8

A methane gas and trimethyl borate were mixed at 1:1 in the vacuumchamber, and the gas mixture above was blown onto a diamond basematerial heated to 1900° C. Here, a degree of vacuum within the vacuumchamber was set to 20 to 30 Torr. Then, graphite containing borondeposited on a substrate. Bulk density of this graphite was 2.0 g/cm³.As a result of ICP element analysis, a concentration of boron ingraphite was 0.5 mass %.

Graphite above was converted to diamond at a synthesis temperature of1800° C. and at 15 GPa, to thereby obtain nano polycrystalline diamondto which boron was added. The polycrystalline diamond had a crystalgrain size from 10 to 100 nm. No precipitation of B₄C was observed inX-ray patterns. This nano polycrystalline diamond had Knoop hardness of120 GPa. A substrate having a size of 3 mm×1 mm was cut from the nanopolycrystalline diamond and electrical resistance of the substrate wasmeasured, which was 10Ω.

Comparative Example 2

Pure graphite having a particle size not greater than 2 μm and B₄C weremixed, the mixture was fired at 2000° C., and a solid solution of carbonwith boron was formed. A concentration of boron in graphite was 0.5 mass%. This graphite was directly converted to polycrystalline diamond at asynthesis temperature of 2200° C. at 15 GPa. The polycrystallinediamond, however, had a crystal grain size from 1 μm to 100 μm, andvariation in crystal grain size was great. This polycrystalline diamondhad Knoop hardness of 75 GPa. A substrate having a size of 3 mm×1 mm wascut from the polycrystalline diamond and electrical resistance of thesubstrate was measured, which was 10Ω.

Comparative Example 3

Pure graphite having a particle size not greater than 2 μm was immersedfor 12 hours in a solution containing boron and thereafter taken out,and graphite was subjected to heating treatment at 2000° C. Aconcentration of boron in graphite after heat treatment was 0.003 mass%. Whether a solution was alkaline, acid, or an organic solvent,substantially no boron was taken into graphite.

Comparative Example 4

In the case that graphite having bulk density of 0.8 g/cm³ was employed,volume change was great, and hence frequency of occurrence of such asituation that an apparatus should inevitably be stopped due to anabnormal condition during synthesis was at least twice.

Comparative Example 5

When non-doped graphite of high purity was employed as a source materialand it was held at a high pressure of 8 GPa at a high temperature of2000° C., graphite did not convert to diamond.

Example 9

Nano polycrystalline diamond in Examples above and non-doped nanopolycrystalline diamond were used to fabricate scribe tools each having4 points at a tip end (having a quadrangular two-dimensional shape),respectively. Each fabricated scribe tool was used to form 200 50mm-long scribe grooves in a sapphire substrate at a load of 20 g.Thereafter, an amount of wear of the polycrystalline diamond at the tipend portion of each scribe tool was observed with an electronmicroscope. Then, the amount of wear of nano polycrystalline diamondsaccording to Examples above was 0.2 time or less, as compared with thatof the scribe tool made of single-crystal diamond.

On the other hand, since non-doped nano polycrystalline diamond waspoorer in characteristics of resistance to thermal oxidation thanboron-added nano polycrystalline diamond in the present example, anamount of wear at the tip end of the scribe tool raised to a hightemperature was 3 times as high as that in the present example. Thus, itcould be confirmed that, by employing nano polycrystalline diamond inExamples above for scribe tools, nano polycrystalline diamond hardlywore, and hence change in tool shape was small and life more noticeablethan that of non-doped nano polycrystalline diamond was exhibited.

Example 10

Nano polycrystalline diamond in Examples above and non-doped nanopolycrystalline diamond were used to fabricate a dresser having a singlepoint at a tip end (having a conical shape). Each fabricated dresser wasworn with a wet method by using a WA (white alumina) grindstone undersuch conditions as a peripheral speed of the grindstone of 30 msec. anda depth of cut of 0.05 mm. Thereafter, an amount of wear of the dresserwas measured with a height gauge, and the amount of wear of nanopolycrystalline diamonds according to Examples above was 0.3 time orless, as compared with that of the dresser made of single-crystaldiamond.

On the other hand, since non-doped nano polycrystalline diamond waspoorer in characteristics of resistance to thermal oxidation thanboron-added nano polycrystalline diamond in the present example, anamount of wear at the tip end of the dresser raised to a hightemperature was 4 times as high as that in the present example. Thus, itcould be confirmed that, by employing nano polycrystalline diamond inExamples above for the dresser, nano polycrystalline diamond hardlywore, and hence change in tool shape was small, and life more noticeablethan that of non-doped nano polycrystalline diamond was exhibited.

Example 11

Nano polycrystalline diamond in Examples above and non-doped nanopolycrystalline diamond were used to fabricate a drill (a rotating tool)having a diameter of ϕ 1 mm and a blade length of 3 mm. Each fabricateddrill was used to drill a 1.0 mm-thick plate made of cemented carbide(WC—Co) under such conditions as revolutions of the drill of 4000 rpmand a feed of 2 μm/revolution. The number of holes that could be drilleduntil the drill was worn or broken was 5 times or more, as compared withthat of the drill made of single-crystal diamond.

On the other hand, since non-doped nano polycrystalline diamond waspoorer in characteristics of resistance to thermal oxidation thanboron-added nano polycrystalline diamond in the present example, anamount of wear at the tip end of the drill raised to a high temperaturewas 4 times as high as that in the present example. Thus, it could beconfirmed that, by employing nano polycrystalline diamond in Examplesabove for the drill, nano polycrystalline diamond hardly wore, hencechange in tool shape was small, and life more noticeable than that ofnon-doped nano polycrystalline diamond was exhibited.

Example 12

Boron-added nano polycrystalline diamond in which an added element wasboron, a concentration thereof was 1×10¹⁸/cm³, and a crystal grain sizethereof was 100 nm was employed as nano polycrystalline diamond used fora wiredrawing die. A wiredrawing die having a hole diameter of ϕ 30 μmwas made of this nano polycrystalline diamond.

The wiredrawing die above was used to evaluate life of a die.Specifically, a wire drawing time period until a hole diameter of thedie having a hole diameter of ϕ 30 μm expanded to ϕ 32 μm was counted,and the wire drawing time period was 5 times as long as the case of useof single-crystal diamond and twice as long as the case of use ofnon-doped nano polycrystalline diamond.

On the other hand, since non-doped nano polycrystalline diamond waspoorer in characteristics of resistance to thermal oxidation thanboron-added nano polycrystalline diamond in the present example, anamount of wear of a wire drawing friction surface raised to a hightemperature was 5 times as much as that in the present example.

It could thus be confirmed that, by employing nano polycrystallinediamond in Examples above for a wiredrawing die, nano polycrystallinediamond hardly wore, hence change in tool shape was small, and life morenoticeable than that of non-doped nano polycrystalline diamond wasexhibited.

Example 13

Boron-added nano polycrystalline diamond in which an added element wasboron, a concentration thereof was 1×10¹⁹/cm³, and a crystal grain sizethereof was 200 nm was employed as nano polycrystalline diamond used foran orifice for abrasive water jet. A polygonal orifice for water jethaving a rectangular shape having a short side of 150 μm and a long sideof 300 μm was made of this nano polycrystalline diamond.

Cutting performance was evaluated by using the orifice for water jetabove. Specifically, a cutting time period until the long side of theorifice for water jet extended to 400 μm was counted, and the cuttingtime period was found to be 20 times as long as that of polycrystallinediamond containing Co as a metal binder and having a diamond particlesize of 5 μm.

On the other hand, since non-doped nano polycrystalline diamond waspoorer in characteristics of resistance to thermal oxidation thanboron-added nano polycrystalline diamond in the present example, anamount of wear at a diamond surface of a hole in contact with abrasiveparticles (hard particles) raised to a high temperature was 3.5 times ashigh as that in the present example. It could thus be confirmed that, byemploying nano polycrystalline diamond in Examples above for a nozzle,nano polycrystalline diamond hardly wore, hence change in tool shape wassmall, and life more noticeable than that of non-doped nanopolycrystalline diamond was exhibited.

Example 14

Graphite was deposited on a substrate with the technique the same as inExample 6. As a result of ICP element analysis, a concentration of boronwas 0.5 mass %, which corresponded to a concentration of boron of1×10²¹/cm³. By making use of this graphite, with the technique the sameas in Example 6, nano polycrystalline diamond was directly obtained fromgraphite. A substrate having a size of 3 mm×1 mm was cut from this nanopolycrystalline diamond and electrical resistance was measured, and anelectrical resistance value was 10Ω.

The conductive nano polycrystalline diamond above was joined to a mainbody of a cutting tool with the use of an active brazing material in aninert atmosphere. After a surface of polycrystalline diamond waspolished, a flank face was cut with electric discharge machining, tothereby fabricate an R cutting tool. For comparison, a tool made ofsintered diamond containing a conventional Co binder (ComparativeExample A) was similarly fabricated with electric discharge machining.

Accuracy of a ridge line of a cutting edge made through electricdischarge machining was from 2 to 5 μm or smaller, depending on aparticle size of contained diamond abrasive grains in ComparativeExample A made of sintered diamond, whereas it was good, that is, notgreater than 0.5 μm, in boron-added nano polycrystalline diamond (thepresent inventive example 1). In addition, a working time period wasalso equivalent to that in Comparative Example A.

In addition, a boron-added nano polycrystalline diamond tool (thepresent inventive example 2) of which flank face was worked withpolishing, a non-doped nano polycrystalline diamond tool (ComparativeExample B), and a single-crystal diamond tool (Comparative Example C)were fabricated and cutting evaluation was made. In both of the presentinventive example 2 and Comparative Example B, accuracy of a ridge lineof a cutting edge was not greater than 0.1 μm and minute accuracy of acutting edge was obtained.

It is noted that contents in evaluation tests are as follows.

-   -   Tool shape: Corner R of 0.4 mm, a relief angle of 11°, and a        rake angle of 0°    -   Work material: Material—aluminum alloy A390        -   Shape—ϕ 110×500 mm with 4 U-shaped grooves    -   Working method: Interrupted turning of an outer circumference of        a cylinder wet method    -   Cutting fluid: Water-soluble emulsion    -   Cutting condition: Cutting speed Vc=800 m/min., depth of cut        ap=0.2 mm, feed rate f=0.1 mm/rev., cutting distance of 10 km

After cutting evaluation was made under the conditions above, thecutting edge of the tool was observed and a state of wear and tear waschecked. Then, in Comparative Example A, an amount of wear of a flankface was as great as approximately 45 μm and a shape of the cutting edgewas lost, whereas in the case of the present inventive example 1, anamount of wear of a flank face was 5 μm, which was satisfactory.

On the other hand, in the present inventive example 2 in which finishingwith polishing was performed, an amount of wear was approximately 2 μm,and it was much better than an amount of wear of 3.5 μm in ComparativeExample B and an amount of wear of 3.5 μm in Comparative Example C. Itwas found that the present inventive example 2 exhibited wear resistancecharacteristics equal to or higher than those of conventional non-dopednano polycrystalline diamond and it was excellent in tool life.

Characteristics were compared between the case of use of non-doped nanopolycrystalline diamond and the case of use of boron-added nanopolycrystalline diamond. Then, non-doped nano polycrystalline diamondburnt owing to oxygen in atmosphere at a high temperature not lower than600° C. and a mass thereof gradually decreased. Then, at 950° C.,deformation of the cutting tool was significant, and 80% of nanopolycrystalline diamond was lost by burning.

In contrast, in nano polycrystalline diamond in Examples above,substantially no decrease in mass was observed even between 600° C. and800° C., and even at a temperature of 950° C. or higher, loss of themass was only 5 to 8%.

It could thus be confirmed that, in the case of use of nanopolycrystalline diamond in Examples above for a cutting tool as well,nano polycrystalline diamond had excellent wear resistance, it was notburnt and lost even at a high temperature, hence change in a tool shapewas small, and it exhibited resistance to oxidation more noticeable thanthat of non-doped nano polycrystalline diamond.

Example 15

For synthesis of source materials for nano polycrystalline diamond,initially, a methane gas and trimethylboron were mixed at 4:1 in thevacuum chamber, and the mixture above was blown onto a diamond substrateheated to 1900° C. to deposit graphite on the substrate. Here, a degreeof vacuum within the chamber was set to 20 to 30 Torr. Bulk density ofthe obtained graphite was 2.0 g/cm³. As a result of SIMS elementanalysis, a concentration of boron was approximately 50 ppm, whichcorresponded to a concentration of boron of 1×10¹⁹/cm³.

Graphite above was made use of to thereby obtain nano polycrystallinediamond directly from graphite at a synthesis temperature of 2200° C.and at 15 GPa. The nano polycrystalline diamond had a crystal grain sizefrom 10 to 100 nm. No precipitation of B₄C or the like was observed inX-ray patterns. This polycrystalline diamond had Knoop hardness of 125GPa. A substrate having a size of 3 mm×1 mm was cut from this nanopolycrystalline diamond and electrical resistance was measured, and anelectrical resistance value was 500Ω.

The conductive nano polycrystalline diamond above was joined to a mainbody of a cutting tool with the use of an active brazing material in aninert atmosphere, and a surface of polycrystalline diamond was polished.In addition, a boron-added polycrystalline diamond tool (the presentinventive example 3) of which flank face was worked with polishing, anon-doped nano polycrystalline diamond tool (Comparative Example D), anda single-crystal diamond tool (Comparative Example E) were fabricatedand cutting evaluation was made. In both of the present inventiveexample 3 and Comparative Example D, accuracy of a ridge line of acutting edge was not greater than 0.1 μm and minute accuracy of acutting edge was obtained.

It is noted that contents in evaluation tests are as follows.

-   -   Tool shape: Corner R of 0.4 mm, a relief angle of 11°, and a        rake angle of 0°    -   Work material: Material—aluminum alloy A390        -   Shape—ϕ 110×500 mm with 4 U-shaped grooves    -   Working method: Interrupted turning of an outer circumference of        a cylinder wet method    -   Cutting fluid: Water-soluble emulsion    -   Cutting condition: Cutting speed Vc=800 m/min., depth of cut        ap=0.2 mm, feed rate f=0.1 mm/rev., cutting distance of 10 km

In the present inventive example 3, an amount of wear was approximately1.0 μm, and it was much better than an amount of wear of 3.5 μm inComparative Example D and an amount of wear of 3.5 μm in ComparativeExample E. It was found that the present inventive example 3 exhibitedwear resistance characteristics equal to or higher than those ofconventional non-doped nano polycrystalline diamond and it was excellentin tool life.

A temperature at a cutting point of a cutting edge of a tool duringcutting is extremely high. Since boron-added nano polycrystallinediamond better in resistance to oxidation than non-doped nanopolycrystalline diamond was extremely unsusceptible to thermaldegradation due to friction heat during cutting or reaction wear at ahigh temperature, it can obtain performance higher than non-doped nanopolycrystalline diamond or single-crystal diamond.

It could thus be confirmed that, in the case of use of nanopolycrystalline diamond in Examples above for a cutting tool as well,nano polycrystalline diamond had excellent wear resistance, hence changein a tool shape was small, and it exhibited performance more noticeablethan non-doped nano polycrystalline diamond.

Example 16

Graphite was deposited on the substrate with the technique the same asin Example 6. As a result of ICP element analysis, a concentration ofboron was 0.5 mass %, which corresponded to a concentration of boron of1×10²¹/cm³. By making use of this graphite, with the technique the sameas in Example 6, nano polycrystalline diamond was directly obtained fromgraphite. A substrate having a size of 3 mm×1 mm was cut from this nanopolycrystalline diamond and electrical resistance was measured, and anelectrical resistance value was 10Ω.

The conductive nano polycrystalline diamond above was joined to a mainbody of a cutting tool with the use of an active brazing material in aninert atmosphere. After a surface of polycrystalline diamond waspolished, a flank face was cut with electric discharge machining, tothereby fabricate a ball end mill (the present inventive example 4) of ϕ0.5 mm, having two twisted cutting blades. For comparison, a tool madeof sintered diamond containing a conventional Co binder (ComparativeExample F) was similarly fabricated through electric dischargemachining.

Accuracy of a ridge line of a cutting edge made through electricdischarge machining was approximately from 2 to 5 μm, depending on aparticle size of contained diamond abrasive grains in ComparativeExample F made of sintered diamond, whereas it was not greater than 0.5μm in boron-added nano polycrystalline diamond (the present inventiveexample 4), which was satisfactory. In addition, non-doped nanopolycrystalline diamond was used to fabricate an end mill having thesame shape with laser machining, and a flank face was locally polishedto finish a cutting edge grade (Comparative Example G).

It is noted that contents in evaluation tests are as follows.

-   -   Tool shape: ϕ 0.5 mm double-bladed ball end mill    -   Work material: Material—STAVAX    -   Cutting fluid: Kerosene    -   Cutting condition: Tool revolution speed of 20000 rpm, depth of        cut ap=0.005 mm/pitch, feed rate f=100 mm/min.

Cutting evaluation was made under the conditions above, and then toollife in the present inventive example 4 was at least 5 times as long asthat in Comparative Example F and at least 1.5 time as long as that inComparative Example G, which was very good. A temperature at a cuttingpoint of a cutting edge of a tool during cutting was extremely high.Since boron-added nano polycrystalline diamond better in resistance tooxidation than non-doped nano polycrystalline diamond was extremelyunsusceptible to thermal degradation due to friction heat during cuttingor reaction wear, it can obtain higher performance than non-doped nanopolycrystalline diamond or single-crystal diamond also in cutting of ahigh-hardness mold material.

It could thus be confirmed that, in the case of use of nanopolycrystalline diamond in Examples above for a rotating tool as well,nano polycrystalline diamond had excellent wear resistance, hence changein a tool shape was small, and it exhibited performance more noticeablethan that of non-doped nano polycrystalline diamond.

Example 17

Graphite was deposited on the substrate with the technique the same asin Example 6. As a result of ICP element analysis, a concentration ofboron was 0.5 mass %, which corresponded to a concentration of boron of1×10²¹/cm³. By making use of this graphite, with the technique the sameas in Example 6, nano polycrystalline diamond was directly obtained fromgraphite. A substrate having a size of 3 mm×1 mm was cut from this nanopolycrystalline diamond and electrical resistance was measured, and anelectrical resistance value was 10Ω. A scribing wheel having a diameterof 3 mm, a thickness of 0.8 mm, and a cutting angle of 120° wasfabricated from the obtained polycrystalline body through electricdischarge machining, and scribing of a glass substrate was evaluated.Consequently, the diamond polycrystalline body in the present examplecould scribe a long distance of approximately 250 km.

For comparison, scribing wheels of the same shape were created withnon-doped polycrystalline diamond and single-crystal diamond, andscribing of a glass substrate was similarly evaluated. Then, non-dopedpolycrystalline diamond could scribe 240 km which was substantiallyequivalent to the diamond polycrystalline body in the present example,whereas single-crystal diamond could scribe only a distance of ⅓thereof.

On the other hand, since non-doped nano polycrystalline diamond waspoorer in characteristics of resistance to thermal oxidation thanboron-added nano polycrystalline diamond in the present example, anamount of wear of an outer circumference of the scribing wheel raised toa high temperature was 1.5 time as high as that of the present example.It could thus be confirmed that, with the use of nano polycrystallinediamond in Examples above for a scribing wheel, nano polycrystallinediamond hardly wore, hence change in a tool shape was small, and lifemore noticeable than non-doped nano polycrystalline diamond wasexhibited.

In addition, it could also be confirmed that excellent tool life, wearresistance, resistance to oxidation, and the like are exhibited byapplying nano polycrystalline diamond in the present example to varioustools.

It is considered, however, that nano polycrystalline diamond havingexcellent characteristics could be fabricated within the scope describedin Scope of Claims for Patent even though conditions are out of therange above.

An embodiment of yet another type of the present invention will bedescribed hereinafter with reference to FIG. 4.

Group-V-element-added nano polycrystalline diamond in the presentembodiment contains a group V element added to be dispersed at theatomic level in carbon forming a polycrystalline diamond body.

A group V element is an element which can have a bond greater by 1 inthe number of electrons than carbon and it is an element serving as adonor in diamond. For example, phosphorus, nitrogen, arsenic, antimony,bismuth, and the like can be exemplified as group V elements. Though oneor more elements selected from theses elements can be employed, otherelements having a similar function may be employed. Among the group Velements, phosphorus is suitable, however, phosphorus alone may be usedor mixed elements which are combination of phosphorus and anotherelement can also be employed.

As shown in FIG. 4, nano polycrystalline diamond 1 in the presentembodiment is formed on base material 2 and contains a group V element 3b uniformly dispersed at the atomic level. It is noted that the “group Velement dispersed at the atomic level” herein refers, for example, to adispersed state at such a level that, when carbon and a group V elementare mixed in a vapor phase state and solidified to thereby fabricatesolid carbon in a vacuum atmosphere, the group V element is dispersed insolid carbon.

Nano polycrystalline diamond 1 can be fabricated by subjecting graphiteformed on the base material to heat treatment. Graphite is an integralsolid and contains a crystallized portion. Though nano polycrystallinediamond 1 has a flat-plate shape in the example in FIG. 4, it ispossible to have any shape and thickness. In the case that nanopolycrystalline diamond 1 is fabricated by subjecting graphite formed onthe base material to heat treatment, nano polycrystalline diamond 1 andgraphite basically have the same shape.

The group V element above can be added to graphite in the stage offormation of graphite. Specifically, graphite can be formed on the basematerial by thermally decomposing a gas mixture of a gas containing agroup V element and a hydrocarbon gas at a temperature not lower than1500° C. so that at the same time, the group V element can be added tographite.

As the gas containing a group V element, for example, a gas composed ofa hydride of a group V element can be employed. In addition, a gas of anorganic metal base which contains a group V element can also beemployed. In the case that phosphorus is added to graphite, a gas of oneor more selected from trimethylphosphorus, triethylphosphorus,trimethylphosphine, triphenylphosphine, and tertiary butyl phosphine canbe employed. In the case that nitrogen is added to graphite, a gas ofone or more selected from trimethyl hydrazine and ammonia can beemployed. In the case that arsenic is added to graphite, a gas of one ormore selected from trimethylarsenic, triethylarsenic, and tertiary butylarsine can be employed. In the case that antimony is added to graphite,a gas of one or more selected from trimethylantimony, triethylantimony,and tertiary butyl antimony can be employed. In the case that bismuth isadded to graphite, a gas of one or more selected from trimethylbismuth,triethylbismuth, and tertiary butyl bismuth can be employed. It is alsopossible that two or more of the gases above are mixed as appropriate.

As described above, by mixing a group V element in a source material gasfor formation of graphite in a vapor phase state to thereby add thegroup V element to graphite, the group V element can uniformly be addedto graphite at the atomic level. In addition, by appropriately adjustingan amount of addition of a gas containing the group V element to thehydrocarbon gas, a desired amount of group V element can be added tographite.

The gas mixture above can thermally be decomposed in a vacuum chamber,and by setting a degree of vacuum within the vacuum chamber to berelatively be high here, introduction of an impurity into graphite canbe suppressed. Actually, however, an unintended inevitable impurity isintroduced in graphite. An element such as hydrogen, oxygen, boron,silicon, and a transition metal, other than the group V element above,can be exemplified as this inevitable impurity.

In graphite used for fabricating group-V-element-added nanopolycrystalline diamond in the present embodiment, an amount of eachinevitable impurity is approximately 0.01 mass % or lower. Namely, aconcentration of an inevitable impurity in graphite is approximately nothigher than the detection limit in SIMS (Secondary Ion MassSpectrometry) analysis. In addition, a concentration of a transitionmetal in graphite is approximately not higher than the detection limitin ICP (Inductively Coupled Plasma) analysis or SIMS analysis.

Thus, in the case that an amount of an impurity in graphite is lowereddown to a level of the detection limit in SIMS analysis or ICP analysisand diamond is fabricated with graphite, polycrystalline diamondextremely small in an amount of impurity other than a group V element ofwhich addition has been intended can be fabricated. It is noted that,even when graphite containing an impurity slightly more than thedetection limit in SIMS analysis or ICP analysis is employed,polycrystalline diamond having characteristics significantly better thanthe conventional example is obtained.

Nano polycrystalline diamond in the present embodiment uniformlycontains a group V element at the atomic level as above, while an amountof an impurity is extremely small. In this nano polycrystalline diamond,atoms of the group V element do not aggregate as clusters in carbon butthey are substantially uniformly dispersed over the entire diamond.Ideally, atoms of the group V element are present as isolated from oneanother in carbon. Atoms of the group V element are present in carbon(the diamond body) in a state substituted for carbon atoms, and they arenot simply introduced in carbon but in such a state that atoms of thegroup V element and carbon atoms are chemically bonded to each other.

As described above, since nano polycrystalline diamond in the presentembodiment contains the group V element dispersed in carbon at theatomic level, nano polycrystalline diamond to which the group V elementhas uniformly been added at an unprecedented level is obtained. Inaddition, since the group V element can uniformly be dispersed in nanopolycrystalline diamond at the atomic level, desired n-type conductivitycan be provided to the entire diamond. Consequently, excellent electronemission characteristics can be provided to diamond.

Here, electron emission characteristics of the group-V-element-addednano polycrystalline diamond in the present embodiment were confirmedand hence results thereof will be described.

A needle-shaped electron gun (an electron emission source) having ashape which can be accommodated in a virtual column having a diameter of6 μm and a height of 25 μm was fabricated and electron emissioncharacteristics thereof were examined. Then, it was found that thiselectron gun could steadily extract an emission current of 150 μA by anextraction voltage of 5 kV and it could be used as a high-performanceelectron source.

In addition, it was also found that this electron gun was composed ofnano polycrystalline diamond, and hence it had no anisotropy and it wasexcellent in durability. In an electron gun made of normalsingle-crystal diamond or a diamond polycrystalline body of a size ofthe micron order, a needle is broken when discharge takes place evenonce. It was found, however, that the electron gun made of nanopolycrystalline diamond in the present embodiment was advantageous as anelectron emission source made of nano polycrystalline diamond in that itdid not break even after discharge 5 to 10 times and change incharacteristics after discharge was also less. Specifically, it could beconfirmed that an emission current was around 150 μA±10 μA by anextraction voltage of 5 kV and change in emission current was verysmall.

From the foregoing, by employing group-V-element-added nanopolycrystalline diamond in the present embodiment for an electron gun,high-performance and stable electron emission characteristics can berealized while durability of the electron gun is improved.

In nano polycrystalline diamond in the present embodiment to which agroup V element has been added, the group V element is dispersed indiamond at the atomic level, and therefore there is substantially nogroup V element introduced in diamond as aggregated as described above.In addition, the added group V element does not aggregate at a crystalgrain boundary of diamond and there is very little impurity in diamond.Therefore, abnormal growth of a diamond crystal can also effectively besuppressed. Consequently, nano polycrystalline diamond having a crystalgrain size (a maximum length of a crystal grain) of a nano size such asfrom 10 to 500 nm and having n-type conductivity is obtained.

In addition, in nano polycrystalline diamond in the present embodiment,concentration distribution of the group V element in diamond is alsoless likely. From this fact as well, local abnormal growth of crystalgrains of diamond can effectively be suppressed. Consequently, ascompared with a conventional example, sizes of the crystal grains ofdiamond can also be the same.

A concentration of a group V element in diamond can arbitrarily be set.A high or low concentration of a group V element in diamond can be set.In any case, since a group V element can uniformly be dispersed indiamond, generation of concentration distribution of the group V elementin diamond can effectively be suppressed. Thus, occurrence of localvariation of conductivity in diamond can also effectively be suppressed.

It is noted that a concentration of an added group V element ispreferably within a range from 10¹⁴ to 10²²/cm³, in order to providen-type conductivity to diamond. In order to provide good conductivitylike a metal to diamond, a concentration of an added group V element ispreferably not lower than approximately 10¹⁹/cm³, and in order toprovide a property as a semiconductor to diamond, a concentration of anadded group V element is approximately from 10¹⁴ to less than 10¹⁹/cm³.

A method for manufacturing group-V-element-added nano polycrystallinediamond in the present embodiment will now be described.

Initially, in a vacuum chamber, a base material is heated to atemperature approximately not lower than 1500° C. and not higher than3000° C. A well known technique can be adopted as a heating method. Forexample, it is possible that a heater capable of directly or indirectlyheating the base material to a temperature not lower than 1500° C. canbe provided in the vacuum chamber.

Any metal, inorganic ceramic material, or carbon material can be used asthe base material, so long as it is a material capable of withstanding atemperature approximately from 1500° C. to 3000° C. From a point of viewof not introducing an impurity in graphite serving as a source material,however, the base material is preferably made of carbon. Morepreferably, it is possible that the base material is composed of diamondor graphite containing very little impurity. In this case, at least asurface of the base material should only be composed of diamond orgraphite.

Then, a hydrocarbon gas and a gas containing a group V element areintroduced in the vacuum chamber. Here, a degree of vacuum within thevacuum chamber is set approximately to 20 to 100 Torr. Thus, thehydrocarbon gas and the gas containing the group V element can be mixedwithin the vacuum chamber. By thermally decomposing this gas mixture ata temperature not lower than 1500° C., graphite in which the group Velement has been taken at the atomic level can be formed on the basematerial. It is noted that the base material may be heated afterintroduction of the gas mixture and then graphite containing the group Velement may be formed on the base material.

For example, a methane gas can be used as the hydrocarbon gas. Variousgases described above can be employed as the gas containing the group Velement. The gas mixture of the hydrocarbon gas and the gas containingthe group V element can be introduced in the vacuum chamber at a ratio,for example, from 10⁻⁷% to 100%.

In forming graphite, the hydrocarbon gas and the gas containing thegroup V element are preferably fed toward the surface of the basematerial. Thus, the gases can be mixed efficiently in the vicinity ofthe base material, so that graphite containing the group V element canefficiently be generated on the base material. The hydrocarbon gas andthe group-V-element-containing gas may be supplied from directly abovethe base material toward the base material, or may be supplied towardthe base material in an oblique direction or in a horizontal direction.It is also possible that a guide member for guiding the hydrocarbon gasand the group-V-element-containing gas to the base material is providedin the vacuum chamber.

Graphite that a group V element has been added to be dispersed in carbonat the atomic level, which is manufactured as described above and has acrystal grain size not greater than 10 μm, is sintered in the vacuumchamber, so that group-V-element-added nano polycrystalline diamond towhich the group V element has uniformly been added at an unprecedentedlevel can be fabricated.

When graphite is thus directly converted to nano polycrystallinediamond, a crystal grain size of graphite is reflected on a crystalgrain size of nano polycrystalline diamond. Then, a crystal grain sizein a crystallized portion of graphite is preferably not greater than 10μm as described above, such that a crystal grain size of the resultantdiamond is of the nanometer order. Thus, after sintering of graphite,nano polycrystalline diamond having crystal grains of a nano size isobtained. For example, polycrystalline diamond can have a crystal grainsize approximately from 10 to 500 nm.

It is noted that, in the step of converting graphite to diamond,graphite is preferably subjected to heat treatment withinhigh-temperature and high-pressure press equipment without adding asintering aid or a catalyst. In addition, in the step of convertinggraphite to diamond, graphite formed on the base material may besubjected to heat treatment within high-temperature and high-pressurepress equipment.

Graphite which can be used for fabrication of nano polycrystallinediamond in the present embodiment is, for example, crystalline graphitepartially containing a crystallized portion or polycrystalline. Densityof graphite is preferably higher than 0.8 g/cm³. Thus, volume changeduring sintering of graphite can be made smaller. From a point of viewof making volume change during sintering of graphite smaller andimproving yield, experimentally, density of graphite is furtherpreferably approximately not lower than 1.4 g/cm³ and not higher than2.0 g/cm³.

The reason why density of graphite is within the range above is becauseit is considered that, when density of graphite is lower than 1.4 g/cm³,volume change during a high-temperature and high-pressure process is toolarge and temperature control may become impossible. In addition, it isbecause, when density of graphite is higher than 2.0 g/cm³, probabilityof occurrence of crack in diamond may be twice or higher.

Examples of yet another type of the present invention will now bedescribed.

Example 18

A methane gas and trimethylphosphorus were mixed at 1:1 in the vacuumchamber, and the gas mixture above was blown onto a diamond basematerial heated to 1900° C. Here, a degree of vacuum within the vacuumchamber was set to 20 to 30 Torr. Then, graphite containing phosphorusdeposited on a substrate. Bulk density of this graphite was 2.0 g/cm³.In addition, according to observation with an SEM (Scanning ElectronMicroscope), graphite had a crystal grain size (a maximum length of acrystal grain) approximately from 100 nm to 10 μm. As a result of ICPelement analysis, a concentration of phosphorus in graphite was 0.06%.

Graphite above was converted to diamond at a synthesis temperature of2200° C. and at 15 GPa in high-temperature and high-pressure pressequipment, to thereby obtain nano polycrystalline diamond to whichphosphorus was added. The polycrystalline diamond had a crystal grainsize from 10 to 100 nm. No precipitation of single-phase phosphorus wasobserved in X-ray patterns. This nano polycrystalline diamond had Knoophardness of 120 GPa. A substrate having a size of 3 mm×1 mm was cut fromthe nano polycrystalline diamond and electrical resistance of thesubstrate was measured, which was 1 kΩ.

Example 19

A methane gas and trimethyl phosphate were mixed at 1:1 in the vacuumchamber, and the gas mixture above was blown onto a diamond basematerial heated to 1900° C. Here, a degree of vacuum within the vacuumchamber was set to 20 to 30 Torr. Then, graphite containing phosphorusdeposited on a substrate. Bulk density of this graphite was 2.0 g/cm³.In addition, according to observation with an SEM (Scanning ElectronMicroscope), graphite had a crystal grain size approximately from 100 nmto 10 μm. As a result of ICP element analysis, a concentration ofphosphorus in graphite was 0.5%.

Graphite above was converted to diamond at a synthesis temperature of2200° C. and at 15 GPa in high-temperature and high-pressure pressequipment, to thereby obtain nano polycrystalline diamond to whichphosphorus was added. The polycrystalline diamond had a crystal grainsize from 10 to 100 nm. No precipitation of single-phase phosphorus wasobserved in X-ray patterns. This nano polycrystalline diamond had Knoophardness of 120 GPa. A substrate having a size of 3 mm×1 mm was cut fromthe nano polycrystalline diamond and electrical resistance of thesubstrate was measured, which was 10Ω.

Example 20

Electron emission characteristics of the nano polycrystalline diamond inExample 19 were examined with the following technique. A needle-shapedelectron gun (an electron emission source) having a shape which can beaccommodated in a virtual column having a diameter of 6 μm and a heightof 25 μm was fabricated. This electron gun can steadily extract anemission current of 150 μA by an extraction voltage of 5 kV. Inaddition, this electron gun did not break even after discharge 5 to 10times, and change in electron emission characteristics after dischargewas also very small, that is, around 150 μA±10 μA at an extractionvoltage of 5 kV, and the characteristics were stable.

Comparative Example 6

Pure graphite having a particle size not greater than 2 μm and redphosphorus were mixed and the mixture was fired at 2000° C., to therebyform a solid solution of carbon with phosphorus. A concentration ofphosphorus in graphite was 0.5%. This graphite was directly converted topolycrystalline diamond at a synthesis temperature of 2200° C. and at 15GPa. In diamond polycrystal, however, an opaque portion and atransparent portion were present, and presence thereof could clearly berecognized even with naked eyes. Polycrystalline diamond had a crystalgrain size from 100 μm to 500 μm and variation in crystal grain size wasgreat. With regard to Knoop hardness of this polycrystalline diamond,the transparent portion (a portion not doped with phosphorus) had Knoophardness of 100 GPa and the opaque portion (a portion doped withphosphorus) had Knoop hardness of 60 GPa. In addition, polycrystallinediamond had electrical resistance of 800 kΩ.

Comparative Example 7

Pure graphite having a particle size not greater than 2 μm was immersedfor 12 hours in a solution containing phosphorus and thereafter takenout, and graphite was subjected to heating treatment at 2000° C. Aconcentration of phosphorus in graphite after heat treatment was 0.001%or lower. Whether a solution was alkaline, acid, or an organic solvent,substantially no phosphorus was taken into graphite.

Comparative Example 8

In the case that graphite having bulk density of 0.8 g/cm³ was employed,frequency of occurrence of such a situation that volume change wasgreat, and hence an abnormal condition during synthesis, that is,deformation of a heater material, was significant, partial or totalbreak of wire occurred, a set value for a current ⋅ a voltage could nolonger be held, and an apparatus should inevitably be stopped was atleast twice.

In Examples above, it could be confirmed that, by setting a degree ofvacuum within a vacuum chamber to 20 to 30 Torr, mixing a hydrocarbongas and a gas containing phosphorus within the vacuum chamber, andsupplying the gas mixture onto a base material heated to a temperaturearound 1900° C., graphite which had a solid phase and a bulk densityaround 2.0 g/cm³ and in which phosphorus was dispersed at the atomiclevel could be fabricated on the base material. In addition, it couldalso be confirmed that, by converting graphite to diamond at a synthesistemperature of 2200° C. and at 15 GPa, nano polycrystalline diamond inwhich phosphorus was dispersed at the atomic level and of which crystalgrain size (a maximum length of a crystal grain) was approximately from10 to 100 nm could be fabricated. It is considered, however, that nanopolycrystalline diamond having excellent characteristics could befabricated within the scope described in Scope of Claims for Patent eventhough conditions are out of the range above.

Though the embodiments and the examples of the present invention havebeen described above, the embodiments and the examples described abovecan also variously be modified. In addition, the scope of the presentinvention is not limited to the embodiments and the examples describedabove. The scope of the present invention is defined by the terms of theclaims and is intended to include any modifications within the scope andmeaning equivalent to the terms of the claims.

The invention claimed is:
 1. A method for manufacturing polycrystallinediamond, comprising the steps of: preparing graphite that a group IIIelement is added to be dispersed in carbon at an atomic level; andconverting said graphite to diamond by subjecting said graphite to heattreatment within high-pressure press equipment; wherein said step ofpreparing graphite includes the step of forming graphite on a basematerial by introducing a gas mixture of a gas containing said group IIIelement and a hydrocarbon gas within a vacuum chamber and thermallydecomposing said gas mixture at a temperature not lower than 1500° C. 2.The method for manufacturing polycrystalline diamond according to claim1, wherein in said step of converting said graphite to diamond, saidgraphite is subjected to heat treatment within said high-pressure pressequipment without adding a sintering aid or a catalyst.
 3. The methodfor manufacturing polycrystalline diamond according to claim 1, whereinin said step of converting said graphite to diamond, said graphiteformed on said base material is subjected to heat treatment at a highpressure not lower than 8 GPa and at 1500° C. or higher.
 4. The methodfor manufacturing polycrystalline diamond according to claim 1, whereinsaid gas mixture is fed toward a surface of said base material.
 5. Themethod for manufacturing polycrystalline diamond according to claim 1,wherein said hydrocarbon gas is a methane gas.